docs: def rm_arrow(self, orig, dest, *args):
          """Remove the :class:`Arrow` that goes from ``orig`` to ``dest``."""
          if (
                  orig not in self.arrow or
                  dest not in self.arrow[orig]
          ):
              raise KeyError("No Arrow from {} to {}".format(orig, dest))
          arr = self.arrow[orig].pop(dest)
          if arr in self.selection_candidates:
              self.selection_candidates.remove(arr)
          self.arrowlayout.remove_widget(arr)